High molecular weight (HMW) microtubule associated proteins (MAPs) appear to be involved in the regulation of microtubule assembly in vitro. Since these proteins correspond to projections observed on the surface of cytoplasmic microtubules, they are also likely to be involved in the interaction of microtubules with other cytoplasmic structures. To investigate the role of the HMW proteins in microtubule assembly and function we will purify the two major classes of HMW proteins referred to as MAP-1 and MAP-2. We will prepare by limited protease digestion a fragment of these proteins which earlier work has indicated to be responsible for binding to microtubules and for promoting microtubule assembly (Vallee, R.B., and Borisy, G.G., 1977, J. Biol. Chem. 252, pp 377-382). We will characterize the fragment with regard to a number of biochemical properties in an attempt to establish the mechanism by which it promotes microtubule assembly. We will also examine the role of the HMW proteins in the interaction of microtubules with other microtubules, with actin filaments, and with intermediate filaments. We will determine whether association of these structures is correlated with the presence of intact HMW projections on the microtubules. We will also attempt to identify the individual MAPs involved in these interactions with particular emphasis on establishing the functional role of MAP-1 and MAP-2.